Perpetual Motion Machine, a device that will run forever without any outside source of energy. Such a machine cannot exist, because it would violate the principle of conservation of energy, which states that energy can neither be created nor destroyed. Some energy would always be wasted (although not destroyed) through friction and so the supply of useful energy—needed to run the machine—would eventually be depleted. Since the machine could not create new energy, it would stop running.
Many attempts have been made to produce a perpetual motion machine, usually involving some natural force. A favorite approach has been to try to use gravity to turn a wheel; the wheel is designed in such a way that the descending side is always heavier than the ascending side. Such a machine can work on paper but never in reality. Electrical and magnetic forces have also been tried with equal lack of success.
Although all available evidence had shown much earlier that such machines could not be constructed, they were not proved to be impossible until 1847, when Hermann von Helmholtz formulated the law of conservation of energy.